It is well recognized that acute rheumatic fever and acute glomerulonephritis are sequelae of Group A streptococcal infection. In developing countries of the tropics and subtropics, rheumatic heart disease is currently the most common form of cardiac damage. Prevalence rates of this disease as high as 22-23 per thousand have been reported in school-age children in urban slums of some developing countries of the world. It is estimated that in India alone, as many as six million children may be afflicted. Although the exact mechanisms of disease causation are not understood, it is clear that rheumatic fever, as well as accute nephritis, follows infection with Streptococcus pyogenes (Group A streptococcus).
The streptococcal M protein is the major virulence factor of this bacterium by virtue of the fact that it imparts to the organism resistance to phagocytic attack. Antigenic variation is the primary mechanism by which the Group A streptococcus is able to evade the host's immune response and thus cause disease in man. Resistance to Group A streptococcal infection is dependent upon the presence of type-specific antibodies to the M protein, a fibrillar molecule found on the surface of the organism. In addition to a number of nontypable strains, about seventy distinct Group A streptococcal M types are currently recognized. Despite the fact that antibodies cross-reactive among certain M types are common, only antibodies prepared against the homologous type are capable of initiating phagocytosis of the organisms (i.e., are opsonic antibodies). Furthermore, not all homologous, or type-specific antibodies are opsonic.
The fact that specific antiserum can be prepared to Group A streptococci has made it possible to detect streptococcal infection by subjecting clinical isolates, such as those obtainable by throat swab, to serological testing. The identification of group A streptococci in an infection requires the isolation of the organism in pure culture, extraction of the group-specific carbohydrate, and reaction with group-specific antiserum. A clinical test for streptococcal infection that could be based upon a property common only to all pathogenic strains would thus be highly desirable.